variety changed to become a game setting patch

[xarick]

The 'Variety distribution' setting which is used for generating the world was a client setting and was not stored in savegames.

This would mean that when I was receiving a map from a multi-player server, this information was not provided. When I was attempting to 'restart' in the console to generate the exact same world, I was getting a slightly different terrain.

This patch makes this setting a game setting, and stores it in savegames. Apparently, this required a savegame version bump.
There is no savegame conversion, since the information wasn't previously stored. Loading a savegame prior to this patch will re-use the value that is currently set in openttd.